Makita 6421 Build Problems

[..X..]

I was able to score a deal (at the time I thought it was) on a "new 6421" that was broken in the box. It looked like someone dropped the box and broke the chain brake ear. So, I had to replace the clutch side case half and decided to throw on an oem 7900 top end assembly.

Well, I have a couple of problems:
1.) I can get the saw to run but It vibrates bad and the throttle is surging like crazy. It almost feels like the vibrating it causing the throttle surging. All the screws are tight so idk what else

2.) My clutch keeps coming loose. I don't know why, I modified a spark plug socket into 3 prongs and put a rope in the cylinder so i can reinstall it. Is there a torque spec? should I use thread locker? Is this causing my severe vibration?

I did watch the clutch end of the crack shaft while rotating the shaft, I couldn't see any wobble in it.

I 'm at a lost.

Thanks

 

[Czed]

@cuinrearview

 

[cuinrearview]

I'm only familiar with the little ones. He posted this same thing on the other site. Kevin mentioned a bent crank. Seemed logical to me.

 

[huskihl]

I’ve got no idea what else it could be. With the light piston in these, they aren’t known for any excessive vibration at all

 

[huskihl]

I mean, the clutch can almost always come loose without a bar and chain on whether it’s been tightened or not.

 

[cuinrearview]

Are there some different length bolts for the AV like on Huskies? Bottoming out somewhere?

 

[Mastermind]

Make sure the key ain't sheared in the flywheel.

 

[Czed]

I was able to score a deal (at the time I thought it was) on a "new 6421" that was broken in the box. It looked like someone dropped the box and broke the chain brake ear. So, I had to replace the clutch side case half and decided to throw on an oem 7900 top end assembly.

Well, I have a couple of problems:
1.) I can get the saw to run but It vibrates bad and the throttle is surging like crazy. It almost feels like the vibrating it causing the throttle surging. All the screws are tight so idk what else

2.) My clutch keeps coming loose. I don't know why, I modified a spark plug socket into 3 prongs and put a rope in the cylinder so i can reinstall it. Is there a torque spec? should I use thread locker? Is this causing my severe vibration?

I did watch the clutch end of the crack shaft while rotating the shaft, I couldn't see any wobble in it.

I 'm at a lost.

Thanks

Pull the plug and chainbrake cover and slowly pull it over
Check for a bent crank.

 

[mortalitool]

all the flywheel fins in place? no missing fins?

did you run the saw with a bar and chain on it? or just the power head only?

 

[mortalitool]

when you replaced the crankcase half, did you relive the tension of the crank in the bearings?
Often times there is stress created and it puts stress on the bearings. A pop on the crank end will relieve this stress and align things back to normal.

 

[..X..]

when you replaced the crankcase half, did you relive the tension of the crank in the bearings?
Often times there is stress created and it puts stress on the bearings. A pop on the crank end will relieve this stress and align things back to normal.

For the first start up I had the bar and chain on.. I notice after I shut off the saw, my chain moved when I tried to start it again. I pulled it apart and my clutch was loose on the shaft.

As far as a pop. No, I guess I didn't know i needed to do that.. lol oh boy

 

[mortalitool]

For the first start up I had the bar and chain on.. I notice after I shut off the saw, my chain moved when I tried to start it again. I pulled it apart and my clutch was loose on the shaft.

As far as a pop. No, I guess I didn't know i needed to do that.. lol oh boy

I gotcha.


Did your crank spin freely after assembly of the bottom end?

 

[mettee]

Take it down to the point you just have the cylinder on the saw, no carb, no exhaust, no spark plug, no clutch, no flywheel, and turn it over by hand.

If it turns over smoothly that is good. If it grabs or binds take note of where it is binding in the stroke of the piston. Remove cylinder and inspect.

Also you can post pics of anything you think might be off.

 

[Creaky limb tree care]

My experience with clutches that keep coming loose, have shown to be a hairline crack along the central threaded part.

 

[dustinwilt68]

Only way to check it right imo is to check the runout of the crank on booth sides, most likely that crank is bent on the PTO side, as others.have said that's what makes sense.

 

[..X..]

Well,after using an actually piston stop (instead of rope) my clutch doesn't come loose and my crazy hand numbing vibration went away. So maybe its safe to say my crank is good...

My next problem is I can't get this thing to run right... I can get it to idle ok when I lean it out,but obviously I can't get throttle, but when I richen the L screw and hit the throttle the saw will still race after I let off on the throttle... Air leak?

 

[Mastermind]

Well,after using an actually piston stop (instead of rope) my clutch doesn't come loose and my crazy hand numbing vibration went away. So maybe its safe to say my crank is good...

My next problem is I can't get this thing to run right... I can get it to idle ok when I lean it out,but obviously I can't get throttle, but when I richen the L screw and hit the throttle the saw will still race after I let off on the throttle... Air leak?

I've lost count of the number of broken piston crowns I've seen from the use of a piston stop. That's why I always use a rope. But hey.....you do whatever you think is best.

As for it not running right....have you pressure tested it?

 

[Stump Shot]

Well,after using an actually piston stop (instead of rope) my clutch doesn't come loose and my crazy hand numbing vibration went away. So maybe its safe to say my crank is good...

My next problem is I can't get this thing to run right... I can get it to idle ok when I lean it out,but obviously I can't get throttle, but when I richen the L screw and hit the throttle the saw will still race after I let off on the throttle... Air leak?

First check that your throttle cable isn't hanging up the throttle for some reason. If not I would pressure/vac test the saw for leaks and identify where from. If there's one place on this saw I could point to I would, the intake manifold boot should probably be a go to, but for the work involved to take it apart to check it, testing is well worth the time first.

 

[..X..]

I've lost count of the number of broken piston crowns I've seen from the use of a piston stop. That's why I always use a rope. But hey.....you do whatever you think is best.

As for it not running right....have you pressure tested it?

And thats why I was using rope, but I wasn't getting a solid stop with the rope I had, and I forgot I was given a plastic piston stop with my tool kit, so i gave that a try.

I haven't yet done a test yet. I need to find gasket material or rubber to block the intake and exhaust port. I know back when I worked on my muscle car, we use to spray carb cleaner around the intake or carb base gasket to find a leak. Not sure if a guy can do that with a 2 cycle motor.

 

[..X..]

First check that your throttle cable isn't hanging up the throttle for some reason. If not I would pressure/vac test the saw for leaks and identify where from. If there's one place on this saw I could point to I would, the intake manifold boot should probably be a go to, but for the work involved to take it apart to check it, testing is well worth the time first.

I didn't think the cable was, but I'll check again.